Question: $ F = \left[\begin{array}{rr}3 & 1 \\ 4 & 2 \\ 3 & 3\end{array}\right]$ $ B = \left[\begin{array}{rr}4 & 5 \\ 3 & -1\end{array}\right]$ What is $ F B$ ?
Because $ F$ has dimensions $(3\times2)$ and $ B$ has dimensions $(2\times2)$ , the answer matrix will have dimensions $(3\times2)$ $ F B = \left[\begin{array}{rr}{3} & {1} \\ {4} & {2} \\ \color{gray}{3} & \color{gray}{3}\end{array}\right] \left[\begin{array}{rr}{4} & \color{#DF0030}{5} \\ {3} & \color{#DF0030}{-1}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ F$ , with the corresponding elements in column $j$ of the second matrix, $ B$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ F$ with the first element in ${\text{column }1}$ of $ B$ , then multiply the second element in ${\text{row }1}$ of $ F$ with the second element in ${\text{column }1}$ of $ B$ , and so on. Add the products together. $ \left[\begin{array}{rr}{3}\cdot{4}+{1}\cdot{3} & ? \\ ? & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ F$ with the corresponding elements in ${\text{column }1}$ of $ B$ and add the products together. $ \left[\begin{array}{rr}{3}\cdot{4}+{1}\cdot{3} & ? \\ {4}\cdot{4}+{2}\cdot{3} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ F$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ B$ and add the products together. $ \left[\begin{array}{rr}{3}\cdot{4}+{1}\cdot{3} & {3}\cdot\color{#DF0030}{5}+{1}\cdot\color{#DF0030}{-1} \\ {4}\cdot{4}+{2}\cdot{3} & ? \\ ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{3}\cdot{4}+{1}\cdot{3} & {3}\cdot\color{#DF0030}{5}+{1}\cdot\color{#DF0030}{-1} \\ {4}\cdot{4}+{2}\cdot{3} & {4}\cdot\color{#DF0030}{5}+{2}\cdot\color{#DF0030}{-1} \\ \color{gray}{3}\cdot{4}+\color{gray}{3}\cdot{3} & \color{gray}{3}\cdot\color{#DF0030}{5}+\color{gray}{3}\cdot\color{#DF0030}{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}15 & 14 \\ 22 & 18 \\ 21 & 12\end{array}\right] $